There are a number of ways. The best option depends on the design requirements of your layout.
The simplest is to perform a find for the specified category and then present the products in a list or table view. You can select the category in a global field formatted with your list of categories, then a script can use that selected value to find all product records of that category.
A portal can also filter to show only related records of a specified category. (And this can be done at the relationship level for older versions of FileMaker.)
A conditional value list can be set that only lists product IDs for a category specified in another field.